X86: allow registers 8-15 in test

This test was failing on some hosts when an unexpected register was used for a
variable. This just extends the regexp to allow the new x86-64 registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182929 91177308-0d34-0410-b5e6-96231b3b80d8